A demand for transmission of high-definition data acquired by an observation satellite to the ground is increasing due to development of a remote sensing technology. Further, a communication service via a satellite is also proposed, and a communication capacity needed between the ground and a satellite is expected to increase in the future. A microwave is currently used as a communication means between the ground and a satellite. However, a communication capacity by a microwave is limited due to tightness of a radio band and a restriction on transmission speed.
Optical space communication is known as a technique capable of dealing with such an increase in transmission capacity. The optical space communication is a technique for communicating by propagating signal light used in optical fiber communication through free space instead of the inside of an optical fiber, and is able to acquire a communication capacity equivalent to that of the optical fiber communication from an idealistic viewpoint. Furthermore, the optical space communication is able to suppress spatial spread of signal light further than that of a microwave, by using signal light having a wavelength in a near-infrared region. Thus, the optical space communication is able to increase the density of a transmission path, which can also increase a communicable data capacity. Further, since signal light has high directivity, an improvement of communication confidentiality is also expected by using the optical space communication.
However, the optical space communication has the following problem as compared to the optical fiber communication. The problem is that a change in traveling direction and spatial pattern of propagating light due to fluctuations in refractive index of the atmosphere of the earth's surface by a temperature change, wind, and the like results in deterioration of communication quality or random fluctuations thereof. For example, when uplink communication from the ground to a satellite is considered, atmospheric fluctuation near a transmitter on the ground may cause a traveling direction of transmitted signal light (transmission light) to make a turn to a random direction, and reception intensity of signal light received by a satellite may greatly fluctuate.
As one of methods of reducing such fluctuations in reception intensity, PTL 1 describes a configuration in which a communication facility on the ground includes a plurality of transmission telescopes. In the configuration described in PTL 1, the plurality of transmission telescopes are spatially disposed away from each other, and thus each signal light transmitted from each of the transmission telescopes is affected by different atmospheric fluctuation. As a result, a probability of incidence of signal light on a reception telescope of a satellite can be increased. Further, a sum of intensity of signal light transmitted from the ground side is increased by increasing the number of transmission telescopes. As a result, reception intensity of signal light of the satellite can be maintained to be high.
Note that an antenna that transmits and receives signal light may be referred to as an “optical antenna” or a “telescope” in the optical space communication. Further, an antenna that transmits signal light may be referred to as a “transmission telescope”, and an antenna that receives signal light may be referred to as a “reception telescope”.
A problem when a plurality of transmission telescopes are used is a delay adjustment between a plurality of beams of transmission lights. Since each transmission light is emitted from an opening of a different transmission telescope one another, a delay due to a difference in optical path length from a modulator inside a transmission device to an emission position occurs. Thus, reception timing of transmission data may vary by each transmission light when the transmission light is received by a satellite, and, in this case, communication quality deteriorates. For such a problem, NPL 1 describes that a delay difference between beams of transmission lights between a transmission telescope and a satellite is eliminated by fixing a plurality of transmission telescopes to a frame. Furthermore, NPL 1 describes that a delay difference between a modulator and a transmission telescope is reduced by measuring a delay amount of light transmission from the modulator to the transmission telescope and adjusting a length of a coaxial cable that transmits modulation information.
Furthermore, PTL 2 describes a delay time measurement method of acquiring, by performing loopback on data transmitted from a radio base station device to a radio transmission/reception device, delay time between both of the devices. PTL 3 describes a configuration in which diversity synthesis is performed on a reception signal by using a plurality of optical receivers.